The present invention relates to a process for producing an element comprising a substrate and at least one antireflection coating having pores, the dimensions of the antireflection coating being below the wavelength of visible light or the neighboring spectral ranges, and also an element produced according to this process having at least one antireflection coating, in particular optical lenses, mirrors or other optical components, such elements having an essentially improved optical antireflection coating.
The use of thin coatings for the antireflection coating of optical components is well-known. To reduce the reflected light at the interface of two optical media having different refractive indices, an optical coating whose refractive index is between that of the two optical media is applied to this interface.
It is further known that this antireflection coating can be improved by using several coatings instead of one coating and alternating coatings having high and low refractive indices. For example, an antireflection coating having three layers reduces the reflectivity of a glass-air interface from 4.3% of the total intensity to less than 1% above the total visible spectrum. Optical antireflection coatings are in particular important for lens systems having several air-glass transitions, since the reflection losses add up at each interface.
Calculating the reflectivity of the interface between two media having the refractive indices n0 and n1 allows one to determine the optical properties of an antireflection coating. A coating whose refractive index equals the root of the product of n1xc3x97n0 is optimum. For monochromatic light the reflectivity drops to 0 when the optical thickness of the coating is one-fourth the wavelength of the radiated light. For air and glass having n0=1 and n1=1.52, for example, the optimum refractive index of an antireflection coating is 1.23. However, the magnesium fluoride material frequently used for antireflection coatings has a refractive index of 1.38 and thus affects a reduction in the reflectivity at the glass-air interface by merely 1.3-2%. Materials having refractive indices lower than 1.3 in the visible wavelength range or in the neighboring infrared or ultraviolet ranges are unknown.
Applying an antireflection coating optimally reduces the reflectivity of the interface at the wavelength corresponding to one-fourth the optical thickness of the antireflection coating. Other wavelengths produce a higher reflectivity of the incident light. The reflectivity can be reduced uniformly in a broader wavelength range with the use of several antireflection coatings. In this case the optical properties of a series of antireflection coatings can also be determined via calculations. Several coatings, each of whose optical thickness equals one-fourth a so-called reference wavelength, are optimum. The refractive index of multicoating systems should ideally vary progressively between the refractive indices of the two media. For a glass-air interface this is not currently feasible with known media, since materials having refractive indices below 1.3 are not available.
A more recent innovation to the described technology consists of the use of so-called nanoporous materials. Materials having pores or air pockets whose dimensions are below the wavelength of visible light have an effective refractive index indicated by the average of the refractive index of the material and that of air. By varying the number of pores per volume and/or by varying the total percent by volume of pores, the refractive index can thus be set continuously by the refractive index of the substrate at a refractive index that is approximately 1. At this time two processes of the prior art which utilize this more recent innovation are known, namely a process that uses the sol-gel method, and a so-called embossing process. However, both processes have the decisive drawback that producing coatings having the desired properties is very expensive and moreover permits only the production of single coatings.
Thus, the problem underlying the present invention is to provide a process which is as simple, fast and affordable as possible for producing such elements comprising a substrate and at least one reflection-reducing coating, such as for example optical lenses, mirrors and other optical components, so that such elements have an essentially improved optical antireflection coating. A further problem of the present invention is to provide an element which in the case of a substrate-(e.g. glass)-air interface has a refractive index below 1.3.
These problems are solved by the embodiments characterized in the claims. In particular, there is provided a process for producing an element comprising a substrate and at least one antireflection coating having pores, the dimensions of the antireflection coating being below the wavelength of visible light or neighboring spectral ranges, comprising the steps
preparing a substrate,
applying a solution of at least two mutually incompatible polymers which are dissolved in a common solvent in such a manner that a common intermixed phase is produced, to a substrate, phase separation on its surface producing a coating having essentially laterally alternating polymer phases, and
exposing this coating to another solvent so that at least one polymer remains undissolved.
When the solution of at least two mutually incompatible polymers is applied in accordance with the process of the present invention, a phase separation takes place in a two-component or multicomponent (xe2x80x9cincompatiblexe2x80x9d) mixture of macromolecular substances which are essentially immiscible, i.e. incompatible. Macromolecular substances suitable for this process are in particular polymers or oligomers, hereafter referred to collectively as polymers. Fundamentally, no restrictions of any kind are placed on the polymers that can be used in the scope of the process in accordance with the present invention, except that in the polymer combinations thus utilized the polymers used are essentially not mutually miscible, i.e. incompatible. Using known polymers in the polymer combinations or mixtures such as e.g. polystyrene, polymethyl methacrylate, polymethacrylate, polyacrylate, polyvinyl pyridine or polyvinyl chloride, all of which have refractive indices around 1.5, antireflection coatings having refractive indices below 1.3, for example even below 1.2, can be produced in accordance with the process of the present invention. Examples of mutually incompatible polymers that can be used are polystyrene and polymethyl methacrylate or polystyrene and polyvinyl chloride. The refractive index of the antireflection coating is the root of the product of the refractive index of the optical substrate and of the neighboring medium.
The process in accordance with the present invention makes it possible to obtain elements of, for example, glass, Plexiglas and polycarbonate having a refractive index less than 1.3 in the case when the neighboring medium is air. For antireflection coatings which consist of several layers or coatings, if one or more layers having a refractive index greater than 1.3 are present, one or more of the additional coatings must have a refractive index less than 1.2 or less than 1.1. If fluorinated polymers, such as e.g. DuPont""s commercially available Teflon AF, having a refractive index of approximately 1.3 are used, antireflection coatings having refractive indices below 1.1 are obtainable according to the process in accordance with the present invention.
The selected polymers are dissolved in a substance that acts as a solvent for all components, such as for example toluene, benzene, tetrahydrofuran, ethanol, acetone and methanol, in such a manner that a common intermixed phase is produced. A coating or film is then produced from this solution on a suitable substrate via spin coating, dip coating, spray coating or other type of application. Assuming that the selected polymers are essentially not mutually miscible, the phase separation occurs while the coating is forming.
The coating produced in such a manner, which has for example a thickness from 50 to 500 nm, preferably 50 to 300 nm, contains after the production two or more phases which are usually present in a lateral arrangement. To obtain an antireflection coating, in an additional step a selective solvent which dissolves only one of the polymers used is used to remove the phase that consists for the most part of this polymer. For a multicomponent mixture this step can be repeated several times as needed. By varying the quantity ratio of the components which are dissolved or broken up in the additional step of the process in accordance with the invention, the effective refractive index of the antireflection layer can be set; i.e., the mixture ratio of the polymers is set as a function of the desired refractive index of the antireflection coating.
Such a separation is known in principle from Walheim et al., Macromolecules, volume 30, pp. 4995-5003 (1997), which describes the separation behavior of a mixture of polystyrene and polymethyl methacrylate from a solvent during the formation of the coating. The separation processes thus described can be regarded as a physical basis for producing antireflection coatings in accordance with the process of the invention in question. However, the dimension of the formed structures lies in the range from 1 to 6 xcexcm; i.e., they are approximately ten times greater than the optical wavelength and are therefore basically not suitable for the antireflection coating of optical lenses, mirrors et al. Optical components coated in this manner actually have a dull appearance, since radiated light is scattered on the produced separation structures.
The optical antireflection coating in accordance with the present invention can be brought about via one or more of the above-mentioned nanoporous polymer coatings produced in accordance with the process of the present invention. In one embodiment of the process in accordance with the invention, the optical antireflection coating is brought about via a nanoporous structure inside the substrate, which is produced using the nanoporous polymer coatings as produced above via, for example, an etching process, the nanoporous polymer coatings formed via the process in accordance with the invention acting as a mask or stencil. Also, when these coatings are produced the dimension of the pores in the phase morphology can be set to be below the wavelength of visible light by choosing suitable parameters.
The antireflection coating formed by the process in accordance with the invention can be arranged, for example, as a single or multiple coating. In a preferred embodiment the at least one antireflection coating can be constructed as an antireflection gradient coating by exposing the coating to a selective solvent only briefly so that no polymer component is completely removed. Such an antireflection gradient coating is preferably constructed in such a manner that the percent by volume of pores or air pockets formed in the coating is a function of the distance from the coating surface, so that the coating has a gradient of the refractive index, so that the refractive index gradually changes from the refractive index of the first medium to that of the second medium. Such a gradient is typically characterized in that the transition from the value of the optical substrate to the neighboring medium is continuous. For an air-glass interface the gradient progresses between values of the refractive index from 1.0 to 1.5. However, gradients having in part such a progression are also possible.
The element obtainable via the process in accordance with the invention can be, for example, an optical lens or a mirror.